Strain gauges measure the change in resistance of an electrical conductor associated with the application of a mechanical load to the conductor which results in a strain. This change in resistance can be due to changes in the geometry of the conductor and in the resistivity of the conductor material. Conventional strain gauges are based on metallic wires or foils or on micromachined single- or polycrystalline semiconductors. These gauges are usually attached to a flexible plastic substrate, which in turn is bonded to the structure for which the strain has to be determined. The sensitivity of a strain gauge is given by the gauge factor “G”, which is defined as the ratio between the relative change in resistance “ΔR/R” and the strain “ε”: G=ΔR/R/ε. For metallic strain gauges, the gauge factor has typical values of about G=2, while for semiconductor gauges, G is considerably higher and can reach values of 70–200. The monograph “Strain Gauge Technology” by Window and Holister describes the prior art in detail. The strain is typically measured in units of “microstrain” (“με”); where 1 με=10−6ε.
Conventional strain gauge technology has a number of disadvantages. Metal wire and foil gauges have a limited sensitivity, which means that rather sophisticated measurement techniques are required to detect relative changes in resistance as low as a few ppm corresponding to low strains in the microstrain range. Furthermore, the resistivity of the conductor material of such gauges is rather low, resulting in increased power dissipation. This places restrictions on the minimum size of the gauge structure and thus on the spatial resolution of the strain measurements. Semiconductor-based gauges, on the other hand, while exhibiting higher sensitivities, suffer from non-linear behaviour, high temperature coefficients of resistivity, low strain limits and a complex (and thus costly) manufacturing process.
Fuchs et al. (U.S. Pat. No. 4,732,042, 1989) have described a strain gauge type based on a thin, granular metal film in which the conduction is governed by the tunnel effect between the metal grains. A theoretical description of this conduction mechanism together with experimental results for granular films of metals such as gold, silver, palladium or tin was given by Abeles et al. (Adv. Phys. 24, 407 (1975)). The granular films are produced using physical or chemical vapour deposition. The sensitivity of granular metal film gauges is comparable to that of semiconductor-based gauges.